Fabrication of integrated circuits includes the formation of patterned layers on a semiconductor wafer that form electrically active regions in and on the wafer surface. As part of the manufacturing process, a masking process referred to as photolithography or photomasking is used to transfer a pattern onto the wafer. Masking involves applying a photoreactive polymer or photoresist onto the wafer by any suitable means such as by spinning of the wafer to distribute liquid photoresist uniformly on its surface. In a typical semiconductor manufacturing process, several iterations of the masking process are employed. Layers of either positive or negative photoresist can be used in various combinations on the same wafer.
Typically, the wafer is heated or “soft baked” such as on a hot plate to improve adhesion of the photoresist to the substrate surface. A photo aligner aligns the wafer to the photomask and then portions of the photoresist coated wafer are exposed to high-energy light so that a pattern is formed as a latent image in the photoresist layer. A developing agent is then applied to develop the portions of the photoresist which were exposed. When positive resist is used, the developed portions of the resist are solubilized by the exposure to high-energy light. Conversely, when negative resist is used, the undeveloped portions of the resist are solubilized. Washing and rinsing steps are carried out that selectively remove the solubilized photoresist. A drying step is carried out.
In the fabrication of semiconductor devices, typically increases in operational speeds of integrated circuits parallel decreases in device feature sizes. As device feature sizes shrink, the thickness of the resist is constant while the width of the pattern decreases. This results in a higher aspect ratio of height to width of photoresist lines. In actual practice, as the aspect ratio increases, the mechanical stability of the resist lines decreases. A serious problem emerges when the mechanical stability of the resist lines is too weak to compensate for capillary forces exerted by the liquid during the drying step. During drying, unbalanced capillary forces exert a net force on the pattern that deforms the resist lines. When the capillary forces exceed the elastic restoring force of the polymer, collapse of the photoresist structure occurs. The collapse of high-aspect-ratio photoresist structures is related to the surface tension of the rinse solution (capillary forces scale with the surface tension of the rinse solution) and is a function of both the density (spacing) and aspect ratio of resist lines. This becomes an increasingly serious problem as device feature sizes continue to shrink while relative vertical height increases to accommodate more complex interconnect structures.
As noted in the literature, collapse of photoresist structures is a generic term that refers to the deformation (bending), fracture, and/or peeling of resist from the substrate, in response to capillary forces present during the drying stage of a lithographic process. D. Goldfarb et. al, Aqueous-Based Photoresist Drying Using Supercritical Carbon Dioxide to Prevent Pattern Collapse, J. Vacuum Sci. Tech. B 18(6), 3313 (2000). Several parameters have been identified which influence the pattern collapse behavior, e.g., the mechanical stiffness of the resist lines are dominated by the Young's modulus (the force per unit cross section of a given substance divided by the fractional increase in length resulting from the stretching of a standard rod or wire of the substance). In addition, due to the different resist chemistries of various vendors, there are different critical aspect ratios of collapse.
A variety of strategies to overcome some of the issues bearing on pattern collapse are published. Conceptually speaking, the simplest method to reduce pattern collapse is to reduce the resist film thickness. However, this method is beginning to show the fundamental limits of the materials constituting the polymeric film. Instead of decreasing the film thickness, a different strategy could be to increase the resist stiffness such as by resist heating during rinsing to harden the resist structures, in order to eliminate or minimize collapse. Another strategy could be to use a supercritical fluid to dry resist patterns after rinsing. Supercritical fluids are characterized by high solvating and solubilizing properties that are typically associated with compositions in the liquid state. Supercritical fluids also have a low viscosity that is characteristic of compositions in the gaseous state. The conventional supercritical fluid drying methods commonly employ alcohol, e.g., ethanol, for rinsing. The ethanol rinse liquid can be directly replaced with carbon dioxide (CO2). However, a strategy of using conventional supercritical fluid drying methods to dry resist patterns would have to overcome the additional problem of water contamination. Typically, resist systems are designed to employ aqueous-based developers and, for some resist systems, water is used for rinsing, for example, after development in an aqueous solution of tetramethyl ammonium hydroxide (TMAH). Moreover, polar organic compounds such as ethanol employed in conventional supercritical drying can not be used to dry water-rinsed resists because they dissolve the resist. When water is used for rinsing, e.g., for resists developed in an aqueous solution of TMAH, the presence of moisture in the atmosphere can not be avoided. This presents a serious problem because moisture in the atmosphere can cause acrylate-type resist to swell and pattern deformation can occur.
The impetus for the recent explorations of supercritical fluid to dry resist patterns is the philosophy that pattern collapse can be minimized by reducing the surface tension of the rinse solution. It is commonly known that one of the mechanisms of pattern collapse is the presence of capillary forces. Moreover, it is known that capillary forces scale with the surface tension of the rinse solution. In mathematical terms, the Laplace equation F=γ/r relates the force (F) acting on the resist walls to the surface tension (γ) of the rinse liquid and the radius (r) of the meniscus in between the patterns. By the equation, decreases in the surface tension relate to decreases in the capillary force acting on the resist walls. D. Goldfarb et. al, Aqueous-Based Photoresist Drying Using Supercritical Carbon Dioxide to Prevent Pattern Collapse, J. Vacuum Sci. Tech. B 18(6), 3313 (2000). Accordingly, there is a need for effective methods for supercritical resist drying to eliminate or minimize the capillary forces present during resist drying.
Two methods of supercritical resist drying using CO2 that were developed for water-rinsed resist patterns are described in H. Namatsu et al., J. Vacuum Sci. Tech. B 18(6), 3308 (2000) (hereinafter, “Namatsu”). As stated in Namatsu, supercritical resist drying in principle should not generate any surface tension. This is because, in the phase diagram for the drying process, the phase does not cross the liquid-vapor equilibrium curve; and consequently, there is no liquid-gas interface where surface tension could be generated. Namatsu, citing, H. Namatsu et al., J. Microelectron. Eng., 46, 129 (1998), and H. Namatsu et al., J. Vacuum Sci. Tech. B 18(2), 780 (2000). In the first method as described in Namatsu, a solution of n-hexane, a CO2-philic liquid (in terms of their solubility in CO2, polymers have been classified as CO2-philic and CO2-phobic) and a surfactant, sorbitan fatty acid ether, first replaces the water and, in turn, is replaced with liquid CO2 before supercritical resist drying (SRD) is performed. In this method, the addition of a compound with a high hydrophilic-lipophilic balance to the surfactant compensates for the poor miscibility of water in a solution of n-hexane and sorbitan fatty acid ether. In the second method, which does not require a CO2-philic liquid, the water is replaced directly with the liquid CO2 containing a surfactant, fluoroether carboxylate, which makes water miscible in CO2, and then SRD is performed.
One disadvantage of the supercritical resist drying methods set forth in Namatsu is that their effectiveness is based on the use of a surfactant to enable rinse water to be replaced with CO2 before the drying step is carried out, resulting in additional chemicals other than CO2 needed for the process. Certain surfactants can dissolve the resist patterns, while various other surfactants can result in the formation of a haze on the surface of the photoresist.
There is a need for effective methods for supercritical resist drying to dry semiconductor wafers with no pattern collapse of the photoresist.